A flat panel display (FPD) serving as a display device has been remarkably advancing in recent years, and various forms of the FPD are superseding the CRT (Cathode Ray Tube) monitors. While the CRT monitor requires a large depthwise dimension, and occupies a large space for setting it up, the FPD can be built thin with significantly reduced depthwise dimension. This allows the FPD to be set up in a space smaller than the space needed for the CRT monitor. Among the various forms of the FPD, a liquid crystal display device (Hereinafter referred to as LCD), in particular, is a forerunning form of the FPD, and remarkable advancement in LCD technology has caused diverse uses of the LCD in various scenes of everyday life, thus attracting more attention to a further advancement of the LCD technology.
However, there still remain some unignorable weaknesses in the LCD yet to be overcome: for example, a response speed and a quality of image reproduction. In order to improve these weaknesses, two technologies are introduced to the LCD.
One of the technologies is called overshoot-driving in which the response speed of liquid crystal (Hereinafter referred to as LC) is compulsively accelerated by applying a greater potential difference to the LC than the general potential difference required for switching LC. Patent document 1 discloses a liquid crystal drive circuit adopting such a overshoot-driving.
Another one of the technologies is called pseudo bit-depth extension, such as dithering, in which a noise pattern is added to increase the level of grayscales. For example, in a case where an LCD adopts an n-bit driver that can handle n-bit data, the noise pattern is added to n-bit grayscales data (2n grayscales (n is an integer)), so that an improved vision seemingly having grayscales of m-bit data (2m grayscales (m is an integer and m>n) is obtained from the n-bit data.
The cost of a LCD driver increases as it handles a larger number of bits. In this view, the pseudo bit-depth extension is an effective solution to realize a LCD capable of displaying a larger number of visible grayscales, thus achieving a high quality of the image reproduction, without a cost increase of the driver. Patent document 2 discloses an example of image display device and image processing device thereof, adopting the pseudo bit-depth extension technology.
Thus, with a combination of the overshoot-driving technology and the pseudo bit-depth extension technology, it is possible to realize an LCD with a high response speed and a high quality of the image reproduction.
As described above, the overshoot-driving boosts signals, and the pseudo bit-depth extension technology adds a noise. Here, when these technologies are combined in order to realize an LCD with a high-response speed and a high quality of the image reproduction, the two technologies must be appropriately combined. In an LCD in which those technologies are inadequately combined, the overshoot-driving may boost noise as well, causing the LCD to output noise-rich images.
(Patent document 1)
    Japanese Patent No. 2708746 (registered on Oct. 17, 1997)(Patent document 2)    Japanese Unexamined Patent Publication No. 2001-337667 (Tokukai 2001-337667; published on Dec. 7, 2001)
In view of the foregoing problem, in a conventional display device, the pseudo bit-depth extension has been carried out after the overshoot-driving is performed. This, however, requires a larger scale of circuit; therefore an increase in the costs for the circuit becomes an inevitable problem.
Firstly, the following provides a little more specific explanation about the pseudo bit-depth extension. In the pseudo bit-depth extension, a signal representing m-bit data (where m>n) is inputted to the LCD from which n-bit data is outputted. A periodical noise pattern is added, by using a circuit, to the upper-n-bit data of the inputted m-bit data, and n-bit data is outputted. This noise pattern, when averaging a certain cycles of it, is generated so as to cause data to become data in m-bit.
In short, by adding the noise pattern to the n-bit data, the n-bit data to which the noise pattern is added indicates pseudo-m-bit grayscales. Thus, in the pseudo bit-depth extension, the m-bit data is inputted, and the n-bit data is outputted. If the overshoot-driving is carried out in a preceding stage of the pseudo bit-depth extension, the overshoot-driving has to be carried out with respect to the m-bit data.
Next, the following is a little more specific explanation on the operation of overshoot-driving. In the overshoot-driving, grayscale data of a first frame is compared with grayscale data of a (1-1) frame. Based on a difference in the respective grayscale data, an amount of data amplification is determined. Here, the (1-1) frame data is data of a preceding frame created by buffering the input data into a frame memory.
Accordingly, in the overshoot driving, an increase in bit-depth of data requires a larger volume of memory.
As a result, the circuit scale needs to be enlarged, thereby increasing the cost. When the overshoot-driving is performed with respect to n-bit data, it simply requires an overshoot-driving circuit with a frame memory enough for storing the n-bit data. However, in the foregoing arrangement, a pseudo bit-depth extension block is arranged in a following stage of the overshoot-driving circuit; therefore, it is required to handle m-bit data in the overshoot driving.
As a result, the frame memory in the overshoot-driving block is enlarged to handle the m-bit data, thus causing the above-mentioned problem of cost rise. Further, in the overshoot-driving, an overshooting parameter for determining the amount of the data amplification is also stored in the form of m-bit. Therefore, the volume of memory for storing the overshooting parameter also increases, thus causing the foregoing problem of cost rise.